250-500 kVA Generator Systems

In short. Between 250 and 500 kVA, generator selection becomes a system design problem. What decides success is the switchgear it feeds, the cables that carry 350 to 700 A, the fault current it contributes, the fuel it needs for the agreed autonomy, the air it must move and the order in which loads come back. Classify loads first, size the set for the backed-up portion, then design the installation around the actual equipment data.

Where the wider system enters the decision

A 250 to 500 kVA set typically serves a factory in one of the industrial areas around Hyderabad, a hospital, a large hotel, a multiplex, a tier-two data room, a cold chain warehouse or a commercial tower. The owner is buying a generating plant with an air circuit breaker, a room or a long canopy, an external fuel tank, a transfer switch rated in hundreds of amperes and a cable installation that costs a meaningful fraction of the set, and each item constrains the others.

Classify the load before sizing

Connected load in an industrial unit is rarely the load the generator must carry. Divide the load list into four groups and the sizing becomes clear.

• Safety loads: fire pumps, emergency lighting, lifts for evacuation, ventilation of hazardous areas. These are mandatory and go on first.
• Critical loads: process steps that are damaged by a stop, cold stores, servers, medical equipment, boiler controls.
• Restart loads: machines that must come back soon, in a defined order, once the critical loads are stable.
• Deferrable loads: comfort cooling, non-essential lines, charging and workshops that can wait for the mains.

The generator is sized for the first three groups with their restart sequence, and the fourth is interlocked off whenever the set is supplying. The load shedding page explains how that interlock is implemented.

Current, breakers and cables

I = (kVA × 1000) / (√3 × 415)

Here kVA is the set rating, √3 is 1.732 and 415 V is the line voltage, so at 415 V a set draws about 1.391 A for every kVA of rating: about 348 A at 250 kVA and about 696 A at 500 kVA. Size the outgoing breaker above that current rather than at it, then size the cable for the breaker rather than for the current, allowing for the derating that the route imposes - grouping, ambient temperature and the length that fixes the voltage drop.

In this expression I is the full-load line current in amperes and kVA is the set rating at 415 V three-phase. A 250 kVA set delivers about 348 A, a 380 kVA set about 529 A and a 500 kVA set about 696 A (illustrative). Those currents move the generator breaker from an MCCB to an air circuit breaker at the upper end, require two to four parallel single-core cables per phase or a busduct, and make voltage drop over a long run a real design constraint. The cable sizing page gives the method and the derating factors for high ambient, grouping and buried routes.

The figures are illustrative planning values; the manufacturer’s data sheet for the specific engine and alternator governs.

Fault level and protection

A generator contributes fault current in proportion to its rating and in inverse proportion to its sub-transient reactance. A 500 kVA alternator with a sub-transient reactance of about 12 percent can feed roughly eight times its rated current into a close fault for the first few cycles, falling quickly as the reactance rises. That is far lower than the DISCOM transformer’s contribution, so protection graded for mains supply may not operate quickly, or at all, on generator supply. Breakers and relays must be checked for both sources, and earth fault protection in particular must be coordinated with the generator’s neutral earthing. The fault current page covers the calculation and the LV switchgear page the equipment choices.

Fuel autonomy

Usable litres required = kW load × specific consumption (L/kWh) × hours

Here kW load is the average electrical load during the outage, specific consumption is the engine’s fuel use per kWh delivered at that load, typically 0.26 to 0.30 L/kWh for engines in this class at 50 to 100 percent load, and hours is the agreed autonomy. A 400 kVA set carrying 240 kW for 12 hours at 0.27 L/kWh needs about 780 litres of usable fuel, so a 990 litre tank is adequate once the unusable bottom layer and the overfill margin are allowed for (illustrative). The base tank under a canopied set rarely holds more than a few hours at this rating, so an external tank with a transfer pump, a level gauge with remote alarm, a bund and a return line is standard. See fuel tank sizing for the method and the storage rules.

Ventilation, exhaust and the room

A 500 kVA set in a room needs an inlet opening and an outlet opening each of several square metres of free area, arranged so that air crosses the engine before reaching the radiator and so that hot discharge air cannot return to the inlet. Where the room cannot provide that, a remote radiator or a ducted discharge with a fan is used, and each adds static pressure that must be checked against the engine fan. The exhaust is a long run of insulated pipe with a silencer and, for a CPCB IV+ set in this class, an aftertreatment unit, all of which must stay within the engine’s back-pressure limit. The generator room ventilation page gives the airflow arithmetic.

Transfer and restart

The automatic transfer switch at this rating is a dedicated four-pole device of 400 to 800 A, either open transition for simple installations or closed transition where the site cannot accept a second interruption when the mains returns. The switch, its controller and the restart sequence are designed together: the critical group transfers first, the restart group follows in timed steps that respect the set’s step-load capability, and the deferrable group is held off. With two sets, the logic also decides which leads and how the second is brought on, the beginning of the paralleling questions on the 500 to 1000 kVA page.

Common mistakes

• Sizing the set from the connected load of the plant. The result is a 500 kVA set carrying 150 kVA for years, with poor fuel economy and carbon build-up.
• Reusing the mains-graded protection settings on generator supply. A fault on a feeder is not cleared quickly because the generator cannot deliver the current the settings assume.
• Treating the tank’s geometric volume as usable fuel. The set stops two hours before the planned autonomy ends.
• Designing the room from the nominal kVA rather than the actual engine’s airflow data. Two 500 kVA sets from different makers can differ widely in radiator airflow, and the louvres end up too small.
• Choosing an open-transition ATS for a site with a continuous process. The second break when mains returns stops the line again.

What to send us

Send the load list classified into safety, critical, restart and deferrable groups, the single-line diagram or a photograph of the main switchboard with its incomer rating, the distance and route from the proposed generator location to the switchboard, the autonomy you need in hours, and photographs or a plan of the room or outdoor location. Include the nameplate of any existing set. Use the generator sales enquiry form and we will return a rating, an installation schematic and a fuel plan.

Questions engineers ask

Generators by rating

• Under 15 kVA generators
• 15–50 kVA generators
• 50–125 kVA generators
• 125–250 kVA generators
• 500–1000 kVA generators
• 1–2 MVA generators
• 2 MVA and above generators

Technical content reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators. Last updated 19 October 2026.